1. Cloned cytotoxic T cells and fresh peripheral blood T cells have been targeted against human tumor cells by coating them with anti-T3 cross linked to anti-tumor cell monoclonal antibodies. Such retargeted T cells lyse cultured human tumor lines with the same specificity as the monoclonal anti tumor antibody. When tested on fresh normal or neoplastic tissue, retargeted human peripheral blood T cells selectively lysed the fresh neoplastic cells. 2. Peripheral blood K cells have been retargeted using anti-FcR cross linked to anti-target cell antibodies. The effector cells are Leu 11+, T3-, and approximately 30% T8+. These same cells also mediate NK activity in vitro, especially when stimulated with II-2. The cytotoxic cells which are targeted with anti-FcR cross linked to anti-target cell antibodies are clearly different from, but have about the same lytic activity as the cells which are targeted with anti-T3 cross linked to anti target cell antibodies. 3. Both retargeted human T and retargeted human K cells show potent anti-tumor activity in Winn type tumor neutralization assays in nude mice. In these studies, the growth of LS174T, a human colon carcinoma was totally blocked at effector: target ratios less than 1:1.